The invention relates to a display device comprising a light source device for generating a light beam, an integrator device for converting the light beam from the light source device into a uniform rectangular light beam, a modulator device for modulating the rectangular light beam from the integrator device with image information to be displayed, and a projection device for projecting the rectangular light beam from the modulator device on a display plane, the integrator device comprising a folding member and an integrator member arranged between the light source device and the folding member.
A display device of this type is known from WO 97/45768.
In the known display device, the integrator member comprises two parallel integrator plates, each having a plurality of lenses arranged in a plane perpendicular to a major axis of the light beam emitted from the light source device, while the folding member comprises a reflecting mirror and a lens and the modulator device comprises a beam-splitting prism, a color-splitting and synthesizing prism, and three reflecting liquid crystal display panels.
By using a folding member, a more compact structure of the display device can be obtained as compared with a structure without a folding member.
Since the reflecting mirror of the folding member is not arranged between the integrator plates of the integrator member, the integrator plates may be arranged closer together and may consequently comprise a larger number of lenses while maintaining a given throughput, which will enhance the image quality of the projected image.
As regards the known display device, it is desirable to cause the light between the liquid crystal display panels and the projection device to be telecentric with a view to the angle dependence of the filter functions of the dichroics used in the color-splitting and synthesizing prism, which is much stronger in glass or synthetic material than in air, and the reflection/voltage characteristics of the liquid crystal display panels.
To obtain this desired telecentricity, the integrator plate which is most adjacent to the reflecting mirror should be arranged in the focal plane of the lens of the folding member.
With a view to the tendency of progressing miniaturization, in which a lamp having a light arc length of 1 mm for the light source device and liquid crystal panels with a diagonal of 2.3 cm (0.9 inch) are considered in 1999, it will be more and more desirable to move the lens of the folding member to a position between said prisms of the modulator device. In this case, however, the use of the reflecting mirror in the folding member inhibits an optimally compact geometry.
It is an object of the invention to improve the known display device in such a way that it has both an optimal compact geometry at a progressing miniaturization and a high display image quality.
To this end, the present invention provides a display device of the type described in the opening paragraph, which is characterized in that the folding member comprises a triangular prism adapted for total internal reflection on at least one or more of its non-folding planes or, for best optical performance, on all of its non-folding planes.
In this way, the triangular prism may operate both as a folding member and as an optical tunnel member so that a tunnel prism is concerned, ensuring that the beam exiting from the tunnel prism is geometrically identical to the beam entering the tunnel prism and, consequently, the exiting beam of the most adjacent integrator plate when using the integrator member with two parallel integrator plates. In other words, the image properties for a window to infinity are the same in the latter case as at the output of the most adjacent integrator plate.
For reasons of costs, an acceptably less than best optical performance may be chosen. Alternatively, the tunnel prism could also be adapted for total internal reflection on its folding plane.
It is to be noted that it has been proposed to modify the known display device in such a way that the reflecting mirror of the folding member is replaced by a triangular prism arranged between, and in contact with, the integrator plates which are then arranged at an angle. However, the result then is that the integrator plates are arranged at a larger distance from each other so that the number of lenses which can be used in the integrator plates at an equal throughput will then be smaller, with the attendant deterioration of the displayed image quality. However, the use of the triangular prism instead of a reflecting mirror between the integrator plates is in itself more favorable because in the first-mentioned case a larger number of lenses proportional to the refractive index of the material of the triangular prism can be used in the integrator plates.